Electromagnetic induction has been known for a long time and it has been used in many applications. In electromagnetic induction a time-varying magnetic flux induces an electromotive force to a closed conductor loop. Vice versa, a time-varying current creates a varying magnetic flux. In transformers, this phenomenon is utilized to transfer energy wirelessly from circuit to another via inductively coupled coils. A primary coil transforms an alternating current into a varying magnetic flux, which is arranged to flow through the secondary coil. The varying magnetic flux then induces an alternating voltage over the secondary coil. The proportion of the input and output voltage can be adjusted by the number of turns in the primary and secondary coils.
Wireless charging is an application where electromagnetic induction is used to transfer energy over air. A wireless charging system comprises a charger device i.e. a power transmitter with a primary coil, and a device to be charged i.e. a power receiver with a secondary coil. The current in the charger device is transferred to the charged device through these electromagnetically coupled coils, and the induced current may be further processed and used to charge the battery of the charged device. Energy is transmitted through inductive coupling from the charger device to the charged device, which may use that energy to charge batteries or as direct power.
A trend in today's charger devices, e.g. in charger devices of portable electronics, is a wireless inductive charger device using an Universal Serial Bus (USB) type wall charger or USB port of some other device, for example, some other portable device, as a power source. These charger devices are suitable to be used in various surroundings, with various devices comprising at least one USB port and without a need to find an electric wall socket for an electric cable of the charger and without a need to connect portable electronics to the charger by a wire. However, some wireless charger devices suitable for wireless charging need 5 W minimum level as input power in order to supply enough power for a device to be charged to ensure effective charging capacity.